In CRC if sender has to send data, It will divide the data bits with g(x) which will give some remainder bits. Those remainder bits are appended to the data bits. Now when this code word is sent to the receiver, the receiver will divide the code word with same g(x) giving some remainder. If this remainder is zero that means data is correct.
Now, If all the systems can communicate with each other does that mean every system in the world have the same g(x). because sender and receiver must have common g(x).
(please answer only if have correct knowledge with some valid proof)
It depends on the protocol. CRC by itself can work for different polynomials, the protocols that use it define the g(x) polynomial to use.
There is a list of examples on https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Standards_and_common_use
This is not an issue since systems cannot communicate using different protocols on the sending and receiving end, obviously. Potentially, a protocol could also use a variable polynomial, somehow decided at the start of the communication, but I can't see why that would be useful
That's a big no. Furthermore, there are several other variations besides just g(x).
If someone tells you to compute a CRC, you have many questions to ask. You need to ask: what is g(x)? What is the initial value of the CRC? Is it exclusive-or'ed with a constant at the end? In what order are bits fed into the CRC, least-significant or most-significant first? In what order are the CRC bits put into the message? In what order are the CRC bytes put into the message?
Here is a catalog of CRCs, with (today) 107 entries. It is not all of the CRCs in use. There are many lengths of CRCs (the degree of g(x)), many polynomials (g(x)), and among those, many choices for the bit orderings, initial value, and final exclusive-or.
The person telling you to compute the CRC might not even know! "Isn't there just one?" they might naively ask (as you have). You then have to either find a definition of the CRC for the protocol you are using, and be able to interpret that, or find examples of correct CRC calculations for your message, and attempt to deduce the parameters.
By the way, not all CRCs will give zero when computing the CRC of a message with the CRC appended. Depending on the initial value and final exclusive-or, you will get a constant for all correct messages, but that constant is not necessarily zero. Even then, you will get a constant only if you compute the bits from the CRC in the proper order. It's actually easier and faster to ignore that property and compute the CRC on just the received message and then simply compare that to the CRC received with the message.
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In bit stuffing why always add non information bits after consecutive 5 bits? Any reason behind that?
Here is some information from tutorialspoint:
Bit-Stuffing: A pattern of bits of arbitrary length is stuffed in the message to differentiate from the delimiter.
The flag field is some fixed sequence of binary values like 01111110. Now the payload can also have similar pattern, but the machine on the network can get confused and misinterpret that payload data as the flag field (indicating end of frame). So, to avoid the machine getting confused, some bits are stuffed into the payload (especially at points where payload data looks like the flag) so as to differentiate it from flag.
I am studying USART, with the help of rs232 and max232 for communication.
I want to know if, in a scenario, sender and receiver are following odd parity and except the parity and start, stop bit rest bits gets swapped. So in this case how the receiver will get to know that data received by the receiver is wrong.Here,
Odd/Even parity is not particularly useful for exactly the reason you have identified - it detects only a subset of errors. In days when the number of gates that could fit on chip was far fewer, it had the advantage at least of requiring minimal logic to implement.
However even if you detect an error, what do you do about it? Normally a higher level packet based protocol is used where the packets have a more robust error check such as a CRC. In that case on an error, the receiver can request a resend of the erroneous packet.
At the word rather then packet level, it is possible to use a more sophisticated error checking mechanism bu using more bits for error checking and fewer for the data. This reduces the effective data rate further, and on a simple UART requires a software implementation. It is even possible to implement error-detection and correction at the word level, but this is seldom used for UART/USART comms.
Background: I've spent a while working with a variety of device interfaces and have seen a lot of protocols, many serial and UDP in which data integrity is handled at the application protocol level. I've been seeking to improve my receive routine handling of protocols in general, and considering the "ideal" design of a protocol.
My question is: is there any protocol framing scheme out there that can definitively identify corrupt data in all cases? For example, consider the standard framing scheme of many protocols:
Field: Length in bytes
<SOH>: 1
<other framing information>: arbitrary, but fixed for a given protocol
<length>: 1 or 2
<data payload etc.>: based on length field (above)
<checksum/CRC>: 1 or 2
<ETX>: 1
For the vast majority of cases, this works fine. When you receive some data, you search for the SOH (or whatever your start byte sequence is), move forward a fixed number of bytes to your length field, and then move that number of bytes (plus or minus some fixed offset) to the end of the packet to your CRC, and if that checks out you know you have a valid packet. If you don't have enough bytes in your input buffer to find an SOH or to have a CRC based on the length field, then you wait until you receive enough to check the CRC. Disregarding CRC collisions (not much we can do about that), this guarantees that your packet is well formed and uncorrupted.
However, if the length field itself is corrupt and has a high value (which I'm running into), then you can't check the (corrupt) packet's CRC until you fill up your input buffer with enough bytes to meet the corrupt length field's requirement.
So is there a deterministic way to get around this, either in the receive handler or in the protocol design itself? I can set a maximum packet length or a timeout to flush my receive buffer in the receive handler, which should solve the problem on a practical level, but I'm still wondering if there's a "pure" theoretical solution that works for the general case and doesn't require setting implementation-specific maximum lengths or timeouts.
Thanks!
The reason why all protocols I know of, including those handling "streaming" data, chop up the datastream in smaller transmission units each with their own checks on board is exactly to avoid the problems you describe. Probably the fundamental flaw in your protocol design is that the blocks are too big.
The accepted answer of this SO question contains a good explanation and a link to a very interesting (but rather heavy on math) paper about this subject.
So in short, you should stick to smaller transmission units not only because of practical programming related arguments but also because of the message length's role in determining the security offered by your crc.
One way would be to encode the length parameter so that it would be easily detected to be corrupted, and save you from reading in the large buffer to check the CRC.
For example, the XModem protocol embeds an 8 bit packet number followed by it's one's complement.
It could mean doubling your length block size, but it's an option.
I'm reading the book Data Communications and Networking 4th Edition Behrouz-Forouzan. I have a question in an exercise that asked me the following: The receiver of a message uses the checksum technique (Checksum) for 8-bit characters and get the following information
100101000011010100101000
. How I can know if the Data sent is correct or not? and why?
I Learned how to calculate the checksum in hexadecimal values, but do not understand as determined by a binary output, if the information is correct.
The sender calculates checksum to the data are sends it with the data in same message.
The receiver calculates the checksum again to the received data and checks if result matches with the received checksum.
There is still a chance that both the data and checksum got modified during transmission so they still match but the likelihood of that happening because of random noise is extremely low.
The Ethernet II frame format does not contain a length field, and I'd like to understand how the end of a frame can be detected without it.
Unfortunately, I have no idea of physics, but the following sounds reasonable to me: we assume that Layer 1 (Physical Layer) provides us with a way of transmitting raw bits in such a way that it is possible to distinguish between the situation where bits are being sent and the situation where nothing is sent (if digital data was coded into analog signals via phase modulation, this would be true, for example - but I don't know if this is really what's done). In this case, an ethernet card could simply wait until a certain time intervall occurs where no more bits are being transmitted, and then decide that the frame transmission has to be finished.
Is this really what's happening?
If yes: where can I find these things, and what are common values for the length of "certain time intervall"? Why does IEEE 802.3 have a length field?
If not: how is it done instead?
Thank you for your help!
Hanno
Your assumption is right. The length field inside the frame is not needed for layer1.
Layer1 uses other means to detect the end of a frame which vary depending on the type of physical layer.
with 10Base-T a frame is followed by a TP_IDL waveform. The lack of further Manchester coded data bits can be detected.
with 100Base-T a frame is ended with an End of Stream Delimiter bit pattern that may not occur in payload data (because of its 4B/5B encoding).
A rough description you can find e.g. here:
http://ww1.microchip.com/downloads/en/AppNotes/01120a.pdf "Ethernet Theory of Operation"